1. Field of the Invention
The present invention relates to a power conversion device incorporating a plurality of power conversion circuits.
2. Description of the Related Art
Power conversion circuits constituted by semiconductor elements are of various types and comprise inverter circuits that convert DC to AC and converter circuits that convert AC to DC. Also, inverter circuits comprise variable voltage variable frequency inverter circuits (hereinbelow termed VVVF inverters) in which the voltage and frequency of the AC output are controlled in variable fashion and constant voltage constant frequency inverter circuits (hereinbelow termed CVCF inverters) in which the voltage and frequency of the AC output are controlled to be fixed; a power conversion system is constituted of these.
Taking a railway vehicle system as an example, a power conversion device incorporating for each control unit a plurality of sets of VVVF inverter circuits that control the induction motors used for vehicle drive is mounted in a vehicle. For example, power conversion devices are commonly employed that incorporate a number of sets of VVVF inverter circuits corresponding to one vehicle, specifically, four sets that respectively individually control a single induction motor. Such a power conversion device is a system having excellent redundancy in that, in the event of malfunction, operation can be continued by isolating the malfunctioning VVVF inverter circuit. CVCF inverter circuits are employed in auxiliary power source systems; recently, a system has been proposed in which redundancy of a railway system is improved by incorporating a plurality of sets of VVVF inverter circuits and a CVCF inverter circuit in a single power conversion circuit; in the event of malfunction of the CVCF inverter circuit one of the sets of VVVF inverter circuits is changed over to the CVCF inverter circuit.
Typically in AC electric trains a system is employed in which an induction motor (inductive electric motor) is driven by controlling output voltage and output frequency using a combination of a converter circuit that converts AC to DC and an inverter circuit that converts DC to AC. Conventionally, these various types of power conversion systems were provided in a vehicle as separate devices but recently there has been a tendency to incorporate the various types of conversion circuit in a single power conversion device with a view to saving installation space and achieving circuit integration in order to reduce the number of wirings connecting the devices. In a power conversion device wherein a plurality of sets of power conversion circuits are accommodated in a single device, these are installed in separate regions defined within the device for, for example, each of the electrical functions of converter conversion circuit section, inverter conversion circuit section, control panel section (control unit) and control power source section, these being often respectively constituted in unit form.
In a conversion circuit section employing semiconductor elements, a cooler is required to discharge the heat generated by the semiconductor elements (hereinbelow referred to as heat loss) to outside the device, to ensure that the semiconductor elements are used with their temperature below the permitted value. The basic construction of the cooler comprises a heat sink section where the semiconductor elements are mounted and a heat discharging section where the heat is dispersed to the atmosphere; the heat sink section (heat receiving section) is placed in a sealed chamber portion of the power conversion device and the heat discharging section is placed in an open chamber section through which the atmosphere passes. The open chamber section where the heat discharging section is placed may project somewhat beyond the body of the device to facilitate dispersal of heat to the atmosphere, or may be constituted as a cooling air channel through which a current of cooling air is forcibly made to flow by an electric fan, or may be arranged in a location exposed to the running air current flowing relatively over the external surface of the device when the vehicle is moving, in the case of a device arranged below the floor of the rail vehicle.
A prior art power conversion device incorporating a plurality of sets of such conversion circuits is described below. FIG. 1 is a circuit diagram of a power conversion device driver of a rail vehicle, being a circuit diagram of a power conversion device incorporating four sets of VVVF inverter circuits that individually control the electric motors for a single vehicle i.e. four induction motors. In this Figure, the positive input terminals of VVVF inverter circuits 4 are connected through circuit breakers 2 and filter reactors 3 with pantograph 1, and the negative input terminals thereof are grounded through the vehicle wheels. Also, respective filter capacitors 5 are connected between the positive and negative input terminals of VVVF inverter circuits 4 and, furthermore, induction motors 6 are connected with the output terminals. Four sets of circuits are connected in parallel in this way, the four sets of VVVF inverter circuits 4 individually controlling the four induction motors in a single vehicle. Of the electrical components shown in this circuit diagram, the four sets of VVVF inverter circuits 4 and filter capacitors are accommodated within a single box to constitute power conversion device 7A, the remaining electrical components being respectively individually separate or accommodated in another box of the device, these being electrically connected by vehicle body wiring to constitute a rail vehicle drive system.
FIG. 2A is a perspective view illustrating the condition in which a power conversion device 7A as described above is mounted below the floor of a rail vehicle; FIG. 2B is a side view seen from the direction of forward movement of the vehicle; FIG. 3A is a plan view of power conversion device 7A seen from the direction in which this is mounted on the bottom face of the vehicle; and FIG. 3B is a bottom face view thereof. In these Figures, power conversion device 7A comprises four coolers that cool semiconductor elements 8 (referring generally to elements constituted by for example connecting snubber diodes in parallel with IGBTs). Semiconductor elements 8 corresponding to one set of VVVF inverter circuits are accommodated together in a single cooler 9 and filter capacitors 5 are accommodated in a single box.
It should be noted that, although a single set of VVVF inverter circuits consists of inverter circuits of the three phases U, V and W, in some cases, a single cooler 9 may be provided in common for each of the phases of four sets of VVVF inverter circuits; thus three of these may be arranged next to each other. With this arrangement, if the semiconductor elements 8 corresponding to each phase constituting the VVVF inverter circuits of each set are thus arranged in integrated fashion, the electrical connection and arrangement of the peripheral components thereof is facilitated, and the power conversion device 7A is functionally divided, making it possible to achieve a unitary construction for each function, so coolers 9 are also of a form in which they are combined in this way for the conversion circuits of each set.
In the cooler 9 shown in FIG. 1 to FIG. 3, there are fitted a cooling units comprising a heat sink section where the semiconductor elements 8 are mounted and a heat discharging section that performs heat dispersal to the atmosphere; the heat sink section is placed in a sealed chamber portion of power conversion device 7A and the heat discharging section is placed in an open chamber section through which the atmosphere passes. The construction is such that the open chamber portion where the heat radiating section is placed protrudes somewhat from the body of the box so as to facilitate dispersal of heat to the atmosphere and furthermore so that it is exposed to the running air current flowing relatively over the external surface of the device during running of the vehicle.
Although heat loss is generated from the semiconductor elements 8 during operation of power conversion device 7A, this is transmitted by thermal conduction to the heat sink sections of the cooling units mounted within coolers 9, so that the semiconductor elements 8 are cooled by heat dispersal to the atmosphere from the heat discharging sections of the cooling units, thereby making it possible for these to be employed at the permitted temperature or below.
Now, during operation in which all of the four sets of VVVF inverters that constitute power conversion device 7A are normal, the four sets of VVVF inverters are individually controlled but, since the heat loss generated from these respective sets is practically the same, practically identical amounts of heat lose are dispersed by the respective coolers 9. However, it is a characteristic of this power conversion device 7A that in the event of malfunction of power conversion device 7A it is arranged for the malfunctioning set of VVVF inverter circuits to be isolated by circuit breaker 2, operation being continued with the remaining sets of VVVF inverter circuits, thus conferring redundancy on the system.
When operation with the remaining three sets of VVVF inverter circuits is continued owing to malfunction of one set of VVVF inverter circuits, it is necessary that a larger current should flow in the semiconductor elements 8 constituting the VVVF inverter circuits that are continuing in operation than would be flowing if all four sets of VVVF inverter circuits were normal. Consequently, the amount of heat loss generated from the semiconductor elements 8 also becomes more than during normal operation, so that a higher performance in respect of heat dispersal capability by coolers 9 is demanded than in the normal case. A heat discharging capability of the respective coolers 9 must therefore be ensured such as to enable not just the heat loss that occurs when all of the four sets of VVVF inverter circuits are operating normally but also the heat loss in the event of malfunction, which is increased compared with normal operation, to be dealt with. In other words, while, in the case of normal operation, there is a margin in respect of the cooling capacity of the respective coolers 9, in the event of malfunction, the cooler 9 where the semiconductor elements 8 of the VVVF inverter circuits that have been isolated due to malfunction are mounted does not perform any discharged heat processing at all, so the coolers 9 where the semiconductor elements 8 of the remaining VVVF inverter circuits, that are continuing in operation, are mounted must perform this heat discharge processing on their own.
As a result, coolers 9 must individually be made of large size and this is a factor impeding miniaturization and weight reduction of the power conversion device 7A.
A further conventional power conversion device incorporating a plurality of sets of power conversion circuits is described below. FIG. 4 is a Circuit diagram of a power conversion device 7B wherein two sets of VVVF inverter circuits for driving a vehicle and a single set of CVCF inverter circuits 17 for serving as a vehicle power source are constituted as a single system; in the Figure, elements that are identical with FIG. 1 are given the same reference symbols and further description thereof is omitted. This is so constructed that it is made capable of performing operation wherein one of the two sets of VVVF inverter circuits is changed over from VVVF inverter circuits to CVCF inverter circuits in the event of malfunction of the CVCF inverter circuits; in this way, redundancy of the vehicle system is improved by guaranteeing the vehicle power source.
FIG. 5A is a perspective view (a bird's-eye view) illustrating the condition in which the power conversion device shown in FIG. 4 is mounted below the floor of a rail vehicle. FIG. 5B is a side view seen from the direction of forward movement of the vehicle, FIG. 6A is a plan view of power conversion device 7 seen from the direction of mounting on the bottom face of the vehicle and FIG. 6B is a bottom face view thereof. Two coolers 9 are provided for the VVVF inverter circuits and a single cooler 9 for the CVCF inverter circuits, in the same way as in the case of the conventional device shown in FIG. 1 to FIG. 3, these being respectively of unitary construction.
Although a detailed description of the changeover operation will not be given here, in the event of malfunction of the CVCF inverter circuits, cooler 9 where the semiconductor elements 8 of the CVCF inverter circuits are mounted, of course, does not perform processing of discharged heat, so the two coolers that previously performed discharged heat processing for the VVVF inverter circuits must now act as coolers for VVVF inverter circuits and CVCF inverter circuits. For the coolers 9 where the set of semiconductor elements 8 are mounted that continue in operation as VVVF inverter circuits, a higher heat discharging capability is demanded than in the case of normal operation, just as in the case of the prior art device shown in FIG. 1 to FIG. 3; thus, an external shape of coolers 9 matching this heat discharging capability is needed. Also, although it is demanded that the VVVF inverter circuits and CVCF inverter circuits should be constructed in common, depending on the vehicle system to which they are applied, the heat loss generated from the semiconductor elements 8 by VVVF inverter circuits and CVCF inverter circuits is not necessarily the same; nevertheless, since coolers 9 of the same shape were employed, it could hardly be said that the respective coolers 9 all bore the burden of heat discharge processing equally, even during normal operation; this fact also was a factor impeding miniaturization and weight reduction of power conversion device 7B.
Yet a further conventional power conversion device incorporating a plurality of sets of conversion circuits is described below with reference to FIG. 7 to FIG. 9. FIG. 7 is a circuit diagram of this power conversion device 7C. This comprises essentially double converter circuits 18 that input AC and convert this AC to DC and inverter circuits 19 that take the DC converted by these converter circuits 18 and convert it to AC controlled with variable voltage and variable frequency; a system that drives four induction motors 6 of a rail vehicle is thereby constituted. FIG. 8A is a plan view seen from the side where this power conversion device 7C is mounted on the vehicle bottom, FIG. 8B is a side view seen from the direction of forwards movement of the vehicle and FIG. 9 is a cross-sectional view seen in the direction of the arrows A—A of FIG. 8B. This power conversion device 7C comprises two cooling units 9a, 9b; of these, cooling unit 9a has mounted thereon semiconductor elements 8 constituting converter circuits while cooling unit 9b has mounted thereon semiconductor elements 8 constituting inverter circuits, respectively; heat discharge is arranged to be performed forcibly by means of a current of air supplied by an electrically driven fan 14.
The heat loss generated from the respective conversion circuits does not increase and decrease with the same timing but rather increases and decreases with different timings. At comparatively low speed of the rail vehicle i.e. during acceleration/deceleration the heat loss generated from the inverter circuits is larger; during comparatively high-speed operation, the heat loss generated from the converter circuits is larger. Consequently, when the heat dispersal from the side of cooling unit 9a is large, the heat dispersal from the side of cooling unit 9b is comparatively small and when the heat dispersal from the side of cooling unit 9b is large the heat dispersal from the side of cooling unit 9a is comparatively small. In addition, since, with this device, the heat discharging sides of cooling units 9a, 9b are forcibly cooled by delivery of an air current, their thermal time constants are small, so the sizes of the respective cooling units 9a, 9b are determined by the maximum heat loss. That is, since the temperature of the coolers tracks increase/decrease of heat loss in a short time, when the heat loss varies with time, it is necessary to make it possible to achieve cooling with the maximum heat loss in a short time.
Even in the case of the power conversion devices shown in FIG. 7 to FIG. 9, just as in the case of the power conversion devices shown in FIG. 1 to FIG. 3 or FIG. 4 to FIG. 6, due to the adoption of a unitary construction for each of the plurality of sets of conversion circuits, an unbalanced condition of the thermal load applied to the respective coolers was frequently produced; as a result, the coolers were of large size and miniaturization/weight reduction of the device was impeded.